The initial temperatures of white dwarf stars, while forming, are of order of \(10^5\,\) K [1]}. They cool down and reach \(2\times 10^4\,\) K after hundred million years. This temperature falls to \(10^4\,\) K over the next billion years [2]}.
Hot white dwarfs are temporal, short-living objects. The oldest observed white dwarfs have temperatures of the order of \(10^3\,\) K [3]}. Recently an ultracold white dwarf 2MASS J0348-6022 with a surface temperature of about \(880\,\) K was observed [4]}.
